Do a Google search for "lawn mower auction (your state's name here) 2005". I add the year so you won't get web articals about auctions of the past decade. Also check your local newspaper for public auctions listed in the classifieds. Once you find them, check out each auctioneer's website for future auctions. Also check with your local parks or public works dept and ask what happens to their old equipment, is it traded in? sold at auction? allowed to rust out in back of the garage? There are treasures out there. In my personal experience, the auction focused on the intricate antique glassware in the house that casually mentions "3 riding mowers" is a possible good deal. I figure 95% of the folks are coming for the fancy blue glass, and I might get a good deal on the icky rusty stuff in the garage.
"Man's mind stretched to a new idea, never goes back to its original dimension." -Oliver Wendell Holmes